SIE.yaml: add auth var unsigned update known acs limitations

The ACS uses very large random blobs to test for unsigned authenticated
variable, which causes failures.

- Introduce a new `KNOWN ACS LIMITATION' category in SIE.yaml.
- Add rules in the SIE.yaml config file to mark those failures as known ACS
  limitations.
- Update the documentation and the schema accordingly.

Reported-by: Sam Moss <sam.moss@arm.com>
Suggested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Suggested-by: Stuart Yoder <stuart.yoder@arm.com>
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>